GaN-based light emitting diodes (LEDs) have been applied and developed for about 10 years. The GaN-based LEDs have significantly advance the LED technologies and has been employed in various applications including full color LED displays, LED traffic signals, white LEDs and the like.
Recently, high-efficiency white LEDs have been expected to replace fluorescent lamps. In particular, the efficiency of white LEDs is approaching that of ordinary fluorescent lamps.
Two major approaches to improve the efficiency of LEDs have been attempted. The first approach is to enhance the internal quantum efficiency determined by the crystal quality and the epitaxial layer structure, and the second approach is to increase the light extraction efficiency.
Since the internal quantum efficiency currently reaches 70˜80%, there is little room for further improvement of the internal quantum efficiency. However, the light extraction efficiency may be further improved. In the light extraction efficiency, it is most important to eliminate internal loss of light due to total internal reflection.
A light emitting diode with improved light extraction efficiency through the prevention of total internal reflection has been disclosed in Applied Physics Letters, Vol. 84, No. 6, pp. 855-857, entitled “Increase in the extraction efficiency of GaN-Based light emitting diodes via surface roughening” by Fujii et al.
The LED is formed by depositing an N-type semiconductor layer, an active layer and a P-type semiconductor layer on a sapphire substrate, bonding the semiconductor layers on a submount, separating the semiconductor layers from the substrate using a laser lift-off (LLO) technique, and then roughening a surface of the N-type semiconductor layer. As the surface of the N-type semiconductor layer is roughened, the extraction efficiency of light emitted to the outside through the N-type semiconductor layer can be improved.
However, since there is a limitation in improvement of the light extraction efficiency, a chip area should be increased in order to obtain necessary light output per unit chip. The increase in the chip area results in the increase in manufacturing costs per chip. Accordingly, a new LED capable of increasing light output from the same unit chip area is required.
Meanwhile, an LED is repeatedly turned on/off according to the direction of a current from an AC power supply. Thus, in a case where the LED is used while connecting directly to the AC power supply, there is a problem in that the LED does not continuously emit light and may be easily damaged due to reverse current.
To solve such a problem of the light emitting diode, a light emitting diode that can be used while connected directly to a high-voltage AC power supply has been disclosed in PCT Publication No. WO 2004/023568 A1 entitled “Light-Emitting Device Having Light-Emitting Elements” by Sakai et al.
According to PCT Publication No. WO 2004/023568 A1, light emitting diodes (light emitting cells) are two-dimensionally connected in series on an insulating substrate such as a sapphire substrate to form LED arrays. Two LED arrays are connected in reverse parallel on the sapphire substrate. As a result, a single chip light emitting device that can be driven by means of an AC power supply is provided.
In such a light emitting device, since LED arrays are alternately operated under an AC power supply, its light output is considerably limited as compared with a case where light emitting cells are simultaneously operated. Thus, it is further necessary to improve the light output per unit area within such a light emitting device.